Comfort Noise Addition for Modeling Background Noise at Low Bit-Rates

1. A decoder being configured for processing an encoded audio bitstream, wherein the decoder comprises: a bitstream decoder configured to derive a decoded audio signal from the bitstream, wherein the decoded audio signal comprises at least one decoded frame; a noise estimation device configured to produce a noise estimation signal comprising an estimation of the level and/or the spectral shape of a noise in the decoded audio signal; a comfort noise generating device configured to derive a comfort noise signal from the noise estimation signal, wherein the comfort noise generating device is configured to create the comfort noise signal based on a target comfort noise level signal; and a combiner configured to combine the decoded frame of the decoded audio signal and the comfort noise signal in order to acquire an audio output signal, in such way that the decoded frame in the audio output signal comprises artificial noise.

2. A decoder according to claim 1 , wherein the decoded frame is an active frame.

3. A decoder according to claim 1 , wherein the decoded frame is an inactive frame.

4. A decoder according to claim 1 , wherein the noise estimating device comprises a spectral analysis device configured to create an analysis signal comprising the level and the spectral shape of the noise in the decoded audio signal and a noise estimation producing device configured to produce the noise estimation signal based on the analysis signal.

5. A decoder according to claim 4 , wherein the control device is configured to determine the energy of the wanted signal of the decoded audio signal based on the analysis signal.

6. A decoder according to claim 1 , wherein the comfort noise generating device comprises a noise generator configured to create a frequency domain comfort noise signal based on the noise estimation signal and a spectral synthesizer configured to create the comfort noise signal based on the frequency domain comfort noise signal.

7. A decoder according to claim 1 , wherein the decoder comprises a switch device configured to switch the decoder alternatively to a first mode of operation or to a second mode of operation, wherein in the first mode of operation the comfort noise signal is fed to the combiner, whereas the comfort noise signal is not fed to the combiner in the second mode of operation.

8. A decoder according to claim 7 , wherein the decoder comprises a control device configured to control the switch device automatically, wherein the control device comprises a noise detector and configured to control the switch device depending on a signal-to-noise ratio of the decoded audio signal, wherein under low-signal-to-noise-ratio-conditions the decoder is switched to the first mode of operation and under high-signal-to-noise-ratio-conditions to the second mode of operation.

9. A decoder according to claim 8 , wherein the control device comprises a side information receiver configured to receive side information comprised in the bitstream, which corresponds to the signal-to-noise ratio of the decoded audio signal, and configured to create a noise detection signal, wherein the noise detector switches the switch device depending on the noise detection signal.

10. A decoder according to claim 9 , wherein the side information corresponding to the signal-to-noise ratio of the decoded audio signal comprises at least one dedicated bit in the bitstream.

11. A decoder according to claim 8 , wherein the control device comprises a wanted signal energy estimator configured to determine an energy of a wanted signal of the decoded audio signal, a noise energy estimator configured to determine an energy of a noise of the decoded audio signal and a signal-to-noise ratio estimator configured to determine the signal-to-noise ratio of the decoded audio signal based on the energy of wanted signal and based on the energy of the noise, wherein the switch device is switched depending on the signal-to-noise ratio determined by the control device.

12. A decoder according to claim 8 , wherein the bitstream comprises active frames and inactive frames, wherein the control device is configured to determine the energy of the wanted signal of the decoded audio signal during the active frames and to determine the energy of the noise of the decoded audio signal during inactive frames.

13. A decoder according to claim 8 , wherein the control device is configured to determine the energy of the noise of the decoded audio signal based on the noise estimation signal.

14. A decoder according to claim 1 , wherein the bitstream comprises active frames and inactive frames, wherein the decoder comprises a side information receiver configured to discriminate between the active frames and the inactive frames based on side information in the bitstream indicating whether the present frame is active or inactive.

15. A decoder according to claim 14 , wherein the side information indicating whether the present frame is active or inactive comprises at least one dedicated bit in the bitstream.

16. A decoder according to claim 1 , wherein the target comfort noise level signal is adjusted depending on a bit-rate of the bitstream.

17. A decoder according to claim 1 , wherein the target comfort noise level is adjusted depending on a noise attenuation level caused by a noise reduction method applied to the bitstream.

18. A decoder according to claim 1 , wherein an energy E w (k) of a frequency band k of the frequency domain comfort noise signal is adjusted depending on the target comfort noise level signal, which indicates a target comfort noise level g tar , for each frequency band k as E w (k)=max {g tar −1)Ê n (k);0}, wherein Ê n (k) refers to an estimate of the energy of the noise of the decoded audio signal at the frequency band k, as delivered by the noise estimation producing device.

19. A decoder according to claim 1 , wherein the decoder comprises a further bitstream decoder, wherein the bitstream decoder and the further bitstream decoder are of different types, wherein the decoder comprises a switch configured to feed either the decoded signal from the bitstream decoder or the decoded signal from the further bitstream decoder to the noise estimation device and to the combiner.

20. An encoder being configured for producing an audio bitstream, wherein the encoder comprises: a bitstream encoder configured to produce an encoded audio signal corresponding to an audio input signal and to derive the bitstream from the encoded audio signal; a signal analyzer comprising a signal-to-noise ratio estimator configured to determine the signal-to-noise ratio of the audio input signal based on an energy of a wanted signal of the audio input signal determined by a wanted signal energy estimator and based on an energy of a noise of the audio input signal determined by noise energy estimator; a comparison device configured to compare the determined sign-to-noise ratio with a threshold; a noise reduction device configured to produce a noise reduced audio signal; and a switch device configured to feed, depending on the result of the comparison of signal-to-noise ratio of the audio input signal, either the audio input signal or the noise reduced audio signal to the bitstream encoder for encoding the respective signal, wherein the bitstream encoder is configured to transmit a side information, which indicates whether the audio input signal or the noise reduced audio signal is encoded, within in the bitstream.

21. A system comprising a decoder and an encoder, wherein the decoder is designed according to claim 1 and/or the encoder is designed according to claim 20 .

22. A method of decoding an audio bitstream, wherein the method comprises: deriving a decoded audio signal from the bitstream, wherein the decoded audio signal comprises at least one decoded frame; producing a noise estimation signal comprising an estimation of the level and/or the spectral shape of a noise in the decoded audio signal; deriving a comfort noise signal from the noise estimation signal and based on a target comfort noise level signal; and combining the decoded frame of the decoded audio signal and the comfort noise signal in order to acquire an audio output signal, in such way that the decoded frame in the audio output signal comprises artificial noise.

23. A method of audio signal encoding for producing an audio bitstream, wherein the method comprises: determining a signal-to-noise ratio of an audio input signal based on a determined energy of a wanted signal of the audio input signal and a determined energy of a noise of the audio input signal; comparing the determined signal-to-noise ratio with a threshold; producing a noise reduced audio signal; producing an encoded audio signal corresponding to the audio input signal, wherein, depending on the result of the comparison of the signal-to-noise ratio of the audio input signal, either the audio input signal or the noise reduced audio signal is encoded; deriving the bitstream from the encoded audio signal; and transmitting a side information, which indicates whether the audio input signal or the noise reduced audio signal is encoded, within the bitstream.

24. A non-transitory computer-readable medium comprising a computer program for performing, when running on a computer or a processor, the method of claim 22 .

25. A non-transitory computer-readable medium comprising a computer program for performing, when running on a computer or a processor, the method of claim 23 .